1. Field of the Invention
The present invention is related to a method for stabilization of the geometric optical beam path of a laser head for the application of precision laser material processing.
2. Description of the Related Art
Controlling the beam path out of a light source is crucial for an optical system. A precision laser processing system requires a precise guidance of the laser beam from the laser head to the target material to be processed. The accuracy will usually determine the precision of the processing. The most common cause of error is the change in the beam path due to a thermally induced opto-mechanical change in the properties of the material used. For a laser cavity or an amplifier, this change can even result in failure of the laser operation or instability in laser performance.
A passive method can be adopted by carefully choosing the optics and opto-mechanical materials, but this will not be a sufficient solution for most precision applications. On the other hand, keeping the temperature stable requires adequate heat control over the entire application system. Such a method often requires expensive and time-consuming maintenance and is not applicable for many laser application systems. Solid-state lasers generating ultrashort pulses e.g., <100 ps, require a constant temperature of the head in order to keep the laser cavity and amplifier in a stable opto-mechanical condition. Moreover, the beam delivery path from the light source to the target material to be processed needs to be taken care of separately.
Laser technology has moved toward alternative design solutions avoiding such temperature control. Fiber-based laser technology is promising in this regard. Indeed, the extreme sensitivity of thermo-optical stability of solid-state lasers has been recognized as a major barrier to the acceptance of short or ultrashort pulse lasers by industry. An active stabilization method is a more viable solution for most laser applications for controlling beam delivery. Prior art known in the precision laser machining community includes devices for beam steering between the laser head and the target material to be processed.
The laser-equipped CNC machine tool in U.S. Pat. No. 6,528,762 B2 automatically maintains precise beam path alignment upon a temperature variation of the environment. For beam control in a laser cavity, U.S. Pat. No. 5,969,851 discloses a method comprising active beam steering in combination with position sensors for the pump laser beam stabilization. In this disclosure the applicable concept is to keep the gain volume in a laser medium overlapped geometrically with the intracavity beam. The parameter for the active beam control is the laser head performance such as the output power, but the geometric beam path out of the laser head cannot be controlled.
From the application system point of view, it is a desirable to use a laser head integrated with beam delivery controlled directly by the application control device. By doing so, design flexibility can be provided to the application designer.
Furthermore, development of optical control devices implemented within a limited physical space is important for viable realization of active beam control. A device utilizing angular sensitivity of total internal reflection can be a solution. A polarization sensitive detection of angular beam movement in reflection or refraction is another solution when laser noise cancellation is critical to enhance detection sensitivity.
Fiber-based laser technology is promising in terms of low maintenance cost, beam quality, high energy efficiency and flexibility of implementation on the manufacturing floor. Regarding beam path control, the advantage of a fiber laser is its inherent thermo-mechanical stability of the beam path in the cavity in comparison to a free-space solid-state laser. The beam path is confined in the fiber and temperature variations cannot change the beam path. The temperature influence on the optical couplers can also be minimized by fusing the fibers of separate components required in the fiber laser, or using miniature fiber-pigtailed optics. No rigorous control of the temperature of the laser system is required except for the pumping source of the gain fiber. Only beam delivery outside the laser cavity to the user target needs to be addressed.